In the formation of integrated circuit devices (ICs), a semiconducting substrate (e.g. silicon) is subjected to a series of chemical and thermal process steps to modify the electrical properties of certain areas of the substrate. Doping is the process of placing specific amounts of dopant atoms into the crystal lattice structure of the substrate or a film deposited on the substrate. In general, the electrical characteristics (e.g. conductivity, resistivity) of a defined region of a semiconductor structure are a function of the concentration and depth of the dopants in that region. In order to obtain electrical devices having predictable and reliable electrical characteristics, a doping process must be accurately controlled to provide an optimal concentration and depth for dopant atoms within the substrate.
One common method of doping is by low pressure chemical vapor deposition (LPCVD). With an LPCVD doping process, a deposition gas and a dopant gas are supplied to a process chamber of CVD reactor. A substrate to be doped and the process chamber are maintained at relatively high temperatures and low pressures. In the process chamber, the deposition gas and the dopant gas thermally decompose and deposit onto the substrate. The deposited film is thus a mixture of a deposition species and a dopant species. Following the deposition process, an annealing step is used to drive the dopant atoms into the crystal lattice structure of the deposited film. The dopant atoms move by filling empty crystal positions (i.e. vacancies) or alternately move through the spaces between the crystal sites (i.e. interstitial).
As an example of a prior art LPCVD doping process, polysilicon thin films may be deposited along with a dopant on a silicon substrate. A suitable deposition gas for depositing polysilicon is silane. In general, the silane decomposes under the vacuum and the high temperature of the process chamber and deposits onto the substrate. Common dopants for polysilicon include phosphorous, boron, and arsenic. Common dopant gases for use in an LPCVD doping process include phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6), and arsine (ASH.sub.3).
Although all of the above noted dopants are important in semiconductor manufacture, the use of arsenic as a dopant in semiconductor manufacture is becoming increasingly important. This is because arsenic doped films, in some instances, exhibit characteristics that are more suitable to the formation of particular semiconductor devices than phosphorus or boron doped films. As an example, the diffusion of arsenic into silicon is an order of magnitude slower than the diffusion of phosphorus into silicon. Diffusion of dopant atoms into areas where they would have a detrimental effect on other device components is thus much easier to control with arsenic as a dopant rather than phosphorus.
There are however, several problems associated with the deposition of arsenic doped films. A major problem with an arsenic doped film deposited with a conventional LPCVD method is that the resistivity of the deposited film is extremely high. As an example, even with a high concentration of arsenic incorporated into the as deposited films (up to 1E20/cm.sup.3) the bulk resistance (Rb) of the film remains high. Further annealing steps reduce the bulk resistance of the deposited films to some extent, but the bulk resistance is still higher than expected for a given level of dopant incorporation.
It is theorized by the present inventors that such a high bulk resistance may result because arsenic in the deposited film is not electrically active. Incorporation of arsenic causes a large amount of microscopic strain in the films and creates microscopic defects. These defects make it difficult to activate arsenic atoms as charge carriers or to compensate for the charge carriers generated by the activated arsenic atoms. A high resistance film thus results. Moreover, the solid solubility limit of arsenic in silicon is an order of magnitude higher than phosphorus in silicon. This property combined with the slower diffusion rate of arsenic into silicon provides less arsenic at the grain boundaries. Again the net result is a higher resistance.
Another problem associated with the LPCVD deposition of arsenic doped polysilicon is that in general, the surface of a deposited film is very rough. Under the process conditions that are most suitable for depositing a low resistivity film, a filament type growth is initiated in the deposited film. Such a rough filamentary growth affects the properties of the deposited film and detracts from the quality and adhesion of subsequently deposited film layers.
Yet another problem associated with the LPCVD deposition of an arsenic doped polysilicon film, is that the deposition rate for depositing such a film is limited. This is because the deposition chemistry for depositing these species is incompatible. The arsenic dopants effectively poison the substrate surface and inhibit further chemical reaction. This drastically lowers the film deposition rate so that long process times are required. Such large process times are unacceptable in a large scale semiconductor manufacturing process.
As is apparent from the foregoing discussion, there is a need in the semiconductor art for an improved LPCVD deposition process for depositing doped films on a substrate. In particular, there is a need for a film deposition process for films and dopant species which have incompatible chemistries (e.g. polysilicon and arsenic) and in which the substrate surface is poisoned by one of the species.
Accordingly, it is an object of the present invention to provide a novel LPCVD method for forming doped thin films and in particular arsenic doped polysilicon films. It is a further object of the present invention to provide a method for forming doped films and particularly arsenic doped polysilicon films that are characterized by a low bulk resistance, a fine grain structure and a smooth surface. It is yet another object of the present invention to provide a method for forming doped films and particularly arsenic doped polysilicon films in which a deposition rate is high and which is suitable to large scale semiconductor manufacture.